1. Field of the Invention
The present invention relates to a surface light source device and, more particularly, to a surface light source device used as a back light, a front light, and a reversible light, for use in a liquid crystal display.
2. Description of the Related Art
(Prior Art 1)
FIG. 1A shows a schematic sectional view of a structure of a conventional liquid crystal display (see Japanese Patent Gazette No. 3151830) and FIG. 1B shows an enlarged view of a part of FIG. 1A. FIG. 2 shows a schematic perspective view of a back light used in the aforementioned liquid crystal display.
A liquid crystal display 11 is constituted by overlapping a liquid crystal panel 13 on the surface side of a back light 12. The back light 12 is one in which a small light source 15 is arranged at a corner portion of a light guide plate 14 and a reflecting plate 16 is arranged facing the backside of the light guide plate 14. Deflection patterns 17, each having a short triangle prism shape arrayed in concentric arcs with a central focus on the position placing the light source 15, are arranged on the backside of the light guide plate 14.
Therefore, when the light source 15 is emitted, light of the light source 15 incident from a light incident surface of the light guide plate 14 to the inside of the light guide plate 14 is radially propagated in the light guide plate 14 while total reflection is repeated between the surface (light emitting plane 18) of the light guide plate 14 and the backside. Light incident to the deflection pattern 17 on this midstream is performed by total reflection at the deflection pattern 17, as marked by a solid arrow shown in FIG. 1A and FIG. 1B. The light performed by total reflection is outputted substantially vertically from the light emitting plane 18 to outside to illuminate the liquid crystal panel 13 from the backside side.
Such the liquid crystal display 11 has features that electric power consumption is low and brightness is high, because light usability is high. Furthermore, two prism sheets are not required to be disposed between the light emitting plane 18 of the light guide plate 14 and the liquid crystal panel 13 and therefore reduction in thickness and reduction in cost of the liquid crystal display 11 can be achieved.
However, in the liquid crystal display 11 of such a structure, as marked by a dash line shown in FIG. 1B, when the light reflected at a flat portion of the backside of the light guide plate 14 in the vicinity of the deflection pattern 17 is entered to the deflection pattern 17 and total reflection is performed there, the light is outputted in a direction largely inclined from the direction perpendicular to the light emitting plane 18. As shown in FIG. 2, when the back light 12 is seen from an oblique direction, a bright line 19 is generated on the surface of the light guide plate 14 due to the light to degrade image quality of the liquid crystal display 11.
(Prior Art 2)
FIG. 3A shows a schematic sectional view of a structure of another conventional liquid crystal display and FIG. 3B shows an enlarged view of a part of FIG. 3A. In order to solve the aforementioned problem, in a liquid crystal display 21 disclosed in Japanese Patent Application No. 2004-045263, a projection 22 having a triangle prism shape is projected on the backside of a light guide plate 14 on the light source side of a deflection pattern 17. When such the projection 22 is provided, light incident to the light source side of the deflection pattern 17 is performed by total reflection at the projection 22 to enter to the deflection pattern 17, as marked by a dash line shown in FIG. 3B. As a result, the light reflected at the deflection pattern 17 is outputted toward a direction substantially perpendicular to a light emitting plane 18 of the light guide plate 14 to be effectively used as illuminated light. Consequently, generation of the bright line 19 as shown in FIG. 2 is suppressed, so that image quality of the liquid crystal display 21 is improved and screen brightness is also improved.
However, in the method in which the projection 22 of the triangle prism shape is projected on the backside of the light guide plate 14, there arises a problem in that when pressure is applied from the reflecting plate 16 side of the backside of the light guide plate 14, the projection 22 is crashed due to the pressure. For example, as shown in FIG. 4, when the liquid crystal display 21 is reversed and is pressed by a metallic rod 23 from above the reflecting plate 16, crush of the projection 22 is observed. During assembly or use of the liquid crystal display 21, there is a case that the liquid crystal display 21 is dropped, and in this case, there is a possibility that the projection 22 of the light guide plate 14 is crushed. Consequently, there is a possibility that brightness decreases at the crushed point of the projection 22 and a bright line is generated. In addition, reference numeral 24 shown in FIG. 4 denotes a diffusion plate.